The invention relates to an antenna device, for example in a radio-frequency identification environment (RFID), to a method for operating the antenna device, and to a radio-frequency identification system.
Passive radio-frequency identification technology is well-known from object identification applications such as logistics. Basically, a radio-frequency identification tag (RFID tag) returns its identification number being stored in its memory when the RFID tag is being queried by means of an RFID reader. Passive RFID tags do not necessitate an internal power supply, as they may obtain the entire needed power from the electromagnetic field being emitted by the reader.
A typical application includes a gate (RFID gate) with an RFID reader and one or more antennas. Objects provided with a tag (e.g. in the shape of pallets) pass through the gate, and their respective IDs are being read out by the reader. In this situation, a typical problem is that several transponders do not identify all objects (i. e., the identification rate is below 100%). Under optimum conditions, current readers only identify 80%-98% of the RFID tags arranged on a pallet, for example. In specific applications, even a rate of 98% is not sufficient.
In order to increase the identification rate of the system, specific techniques such as antenna beam forming may be used. This method necessitates that the antenna characteristic is being controlled by means of electronics (e.g. a microcontroller, radio-frequency switches, etc.) which leads to the antennas becoming “active”.
Usually, RFID readers do not supply a power connection for this type of antennas, and so the power has to be supplied externally, i. e., by means of a voltage supply or a battery. The fact that these “smart” antennas involve an external voltage supply or a battery may be a reason of rejection for potential customers and users, as the readers, for example, have to be modified or bought, which incurs costs, or maintenance (such as changing batteries) is involved, which also incurs costs.
FIG. 8 shows a schematic block diagram of an RFID reader with an active RFID antenna 80 according to conventional technology. The active RFID antenna 80 is connected to an RFID reader 122 and configured to receive an RFID signal to be transmitted 124 from the RFID reader 122. The active antenna 80 comprises a controlled (smart) antenna structure 126 (smart antenna) that is supplied with power by means of a battery or an external power supply 128. The active RFID antenna 80 is configured to transmit the RFID output signal, i. e. the radio signal 132.